updated spec: @import (#16808)
updated spec: @import (#16808)

The spec was pointing to version #3. We are at parts of #5 being implemented. 
Cascade layers is in #5
https://www.w3.org/TR/css-cascade-5/#at-import
mdn/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data